1. Field of the Invention
The invention is related to methods of manufacturing of semiconductor materials and devices, and more particularly, to manufacturing non-polar epitaxial heterostructures of third group elements nitrides (further A3N structures) by Organometallic Vapor-Phase Epitaxy (further OMVPE) which are usually used for such devices, as lasers, light emitting diodes (LEDs), and particularly, white LEDs.
2. Discussion of the Background
A3N semiconductor heterostructures are basic materials for design and manufacture of high efficient light emitting diodes and lasers in visible and ultraviolet parts of optical spectrum of radiation, including white LEDs.
In the reference [1] use of converting dark blue and/or ultra-violet radiation of GaN-mis structures into longer wavelength radiation in visible part of spectrum with the help of covering these structures by stocks phosphors was offered for the first time.
In the reference [2] design of white light emitting diodes on the basis of dark blue p-n AlGaInN heterostructure emitters covered by Yttrium-Aluminum-Garnet phosphor has been offered. Part of the primary dark blue radiation of emitters is converted into yellow radiation of phosphor. As a result, mixing of blue radiation from an emitter and complementary yellow luminescence exited by the blue radiation in phosphor produce white light by LEDs with certain coordinates of chromaticity.
Three basic designs of white light-emitting diodes essentially differing from each other are known:                light-emitting diodes on the basis of an emitter of dark blue color of luminescence which is covered by a layer of stocks phosphor converting a part of dark blue radiation into yellow radiation;        light-emitting diodes on the basis of an emitter of ultraviolet radiation which is covered by a layer of stocks phosphor converting ultraviolet radiation into red, green and dark blue bands of luminescence (RGB system);        full-color light-emitting diodes containing three separate emitters radiating in red, green and dark blue parts of spectrum (RGB system).        
Despite of distinction, improvement of parameters of all listed types of white light-emitting diodes demands perfection of methods of epitaxial A3N-heterostructures growth and increase of quantum output of radiation of phosphors.
For mass production of light emitting diodes the most preferable method of manufacturing A3N-heterostructures is the method of Organometallic Vapor-Phase Epitaxy (OMVPE).
Sapphire (Al2O3), silicon carbide (6H—SiC), gallium nitride (GaN) and aluminum nitride (AlN) are used as substrates for A3N epitaxial structures growth. Cheaper sapphire substrates are most of all used. Silicon carbide substrates in some times more expensive than sapphire ones and, therefore, are used not so often. Close to ideal there could be substrates made of GaN or AlN, but their mass production is not achieved yet. Typical A3N-heterostructures for light-emitting diodes contain following functional parts:                a single crystal substrate of sapphire or silicon carbide which surface is crystallographic c-plane (0001) defining crystallographic type of A3N epitaxial layers, for example, wurtzite type of their crystal structures and azimuthally orientation of crystallographic lattices;        wide-bandgap emitters, as a rule, n-type and p-type AIxGa1-XN layers providing effective injection of electrons and holes and their confinement in active region of the heterostructure;        an active region containing, as a rule, a set of narrow-bandgap layers of such materials, as InxGatXN alloys which are usually not specially doped;        contact epitaxial GaN layers of n-type and p-type conductivity providing low specific resistance of ohmic contacts and uniform distribution of current density in a cross-section of a device.        
In A3N-epitaxial heterostructures used in various devices, in particular in light-emitting diodes and lasers, density of defects (dislocations, defects of packing, etc.) and also a level of mechanical stresses should be as low, as possible. For example, GaAs laser heterostructures usually have dislocation density not exceeding values of 102-103 cm−2. In A3N-heterostructures basically exists two sources of defects, first of which concerns to a difference of lattice parameters of a substrate and A3N epitaxial layers and second one concerns to a mismatch of lattice parameters of layers inside of a heterostructure, for example, between GaN and AIxGa1-XN layers or between GaN and InxGa1-xN layers. In the case of GaN or AlN substrates the contribution of the first defects source is decreasing and is comparable with the second defects source contribution.
A3N single-crystal epitaxial layers which have wurtzite type of crystal structure: AlN (lattice parameter a=0.311 nm), GaN (a=0.316 nm) and InN (a=0.354 nm), grown on single-crystal Al2O3 substrates oriented in (0001)-plane (the oxygen sublattice parameter a=0.275 nm) or on 6H—SiC substrates (a=0.308 nm), always contain high density of defects, basically dislocations.
Dislocations are formed in interface “substrate-epitaxial layer” because there is an essential difference of lattice parameters of a substrate and a epitaxial layer. Lattice parameters of epitaxial layers are larger than a lattice parameter of a substrate (discrepancy up to 16%) and dislocations will spread through heterostructure layers. In typical AlGaInN heterostructures used in blue and green light-emitting diodes, which have been grown on sapphire substrates, dislocation densities may have values 108-1010 cm−2. For similar heterostructures grown on SiC substrates dislocation densities may have values 107-109 cm−2. Thus, the contribution of the first source of defects is defined by a value 107-109 cm−2, the contribution of the second source of dislocations formation inside a heterostructure is equal to 106-107 cm−2. In particular, formation of high density of dislocations and even cracking AlGaN layers is caused by a difference of lattice parameters of GaN and AlN layers (discrepancy of 3.5%) and by their differences in thermal expansion coefficient values.
For the partial solution of these problems can be used methods. In first of them before growing a AlGaN layer, for example, n-type emitter layer, a thin In0.1Ga0.9N layer is grown (thickness about 0.1 microns) to prevent cracking a subsequent AlxGa1-xN (x=0.15-0.20) layer. In the second method instead of a bulk AlxGa1-xN n-type emitter layer with a constant x-value a strained multiquantum superlattice AlGaN/GaN layer is grown. The thickness of each layer in the superlattice is about 0.25 nm.
A very special feature of Organometallic Vapor-Phase Epitaxy for A3N-heterostructures growth is necessity of abrupt changing temperature of substrates during a technological process. So, at growing a buffer layer (usually a very thin amorphous GaN or AlN layer) the temperature of sapphire or silicon carbide substrates is rapidly decreased from 1050° C.-1100° C. down to 550° C. and after finishing the amorphous GaN or AlN layer growth the substrate temperature is rapidly increased up to the temperature of growth of a single crystalline GaN layer (1050° C.). If process of heating substrates with a buffer GaN or AlN layer is slow, it will lead to crystallization of a thin (about 20 nm) GaN layer and subsequent growing a thick GaN layer leads to formation of a nonplanar film which has great number of defects and figures of growth.
Another necessity of change of substrate temperature during growth is realized at growing InxGa1-xN layers (at x>0.1) in active region of the heterostructure. These layers have a tendency to thermal decomposition at temperatures above 850° C.-870° C. In this case growing InxGa1-xN layers is completed at a lower (800° C.-850° C.) temperature. During increasing the substrate temperature up to 1000° C.-1050° C. the process of heterostructure growth should be interrupted by disconnecting submission of metalloorganic Ga, Al and In precursors to substrates. With the purpose to exclude thermal decomposition of InxGa1-xN layers they are sometimes covered with a thin (−20 nm) protective Al0.2Ga0.8N layer. This layer has sufficient stability to dissociation up to temperatures about 1050° C. Sharp change of temperature of a substrate with deposited epitaxial layers (except during a GaN or AlN buffer GaN layer growing) can lead to additional formation of defects and cracking grown layers, for example, AlGaN layers. Thus, it is desirable to have such methods of A3N-heterostructures growth, in particular structures for super bright light-emitting diodes, which allow smooth change of growth temperatures and exclude interruptions of a growth process at InxGa1-xN layers growing. These methods of growth have also to reduce density of dislocations generated in interfaces of A3N heterostructure layers. Reduction of dislocations penetrating into a (0001) heterostructure grown on sapphire or silicon carbide substrates can be achieved by use of special techniques including lateral epitaxial overgrowth (LEO-technology). At first, in this technology a thin buffer GaN layer is usually grown at a low temperature. Then a SiO2 or Si3N4 film is deposited on the structure surface. In this film narrow long parallel each other windows are etched down to the buffer layer and then, during the next epitaxy process, a thick GaN layer has been grown on SiO2 or Si3N4 film surface at a high temperature. In the same process a A3N heterostructure is also grown up. It is easy to see, that the LEO-technology is much more complex and more labour-consuming, than usual technology.
Theoretical and, partially, experimental investigations predict advantage of use non-polar a-plane (further a-A3N) heterostructures in a lot of devices, in particular, in light-emitting diodes and lasers. In comparison with usual polar heterostructures grown along the polar c-direction [0001] in a-A3N non-polar heterostructures strong electrostatic fields along the direction of growth are absent. Owing to it, spatial separation of injected electrons and holes in the active region of non-polar a-A3N heterostructures is eliminated and, as consequence, increase of internal quantum efficiencies of radiation in light-emitting diodes and lasers made on their basis can be expected.
A lot of publications is devoted to growth of a-A3N non-polar heterostructures. In the patent application [3] growth of a-GaN (1120) films on r-plane (1102) sapphire substrates is described. In the publication [4] advanced a-A3N non-polar hetero structures grown on a-GaN substrates are proposed by Sh. Nakamura.
At last, in the patent application [3] the opportunities of a-A3N non-polar heterostructures growth on silicon carbide, silicon, zinc oxide, lithium aluminates, lithium niobate and germanium substrates are mentioned.
Thus, a-A3N non-polar heterostructures growth providing low dislocations and structural defects densities is rather actual direction of technology developments to solve problems of increasing internal quantum efficiencies of light-emitting diodes and lasers and their life-times.